Add option to start training paused

[aayushg55]

--start_paused option for ns-train to set initial training state as paused

Closes #3358

[jb-ye]

Could you elaborate how you test this change locally?

[aayushg55]

Starting training with something like ns-train nerfacto --start_paused=True --data data/nerfstudio/poster, training will not immediately start and the webviewer will have the training button in the Resume Training state instead of Pause Training. The training button can then be used to toggle training as usual. Same applies to the legacy viewer.